It is generally known to fit the outlet of a molten-steel vessel with a slide gate and control the flow rate.
A slide gate consists mainly of a pair of refractory plates, which constitute a gate, and a mechanism to support and drive the refractory plates. One plate is moved to adjust the opening degree of the gate in order to control the flow rate of molten steel. It is necessary to effect the surface pressure between the refractory plates to prevent the static pressure of molten steel from causing the leak of molten steel from between the refractory plates. If the surface pressure is inadequate and molten steel leaks, it may lead to a serious accident.
The surface pressure between the refractory plates of an ordinary slide gates is several tenths of 1 MPa and, hence, the whole surface pressure is 2 to 10 tonf. On the other hand, because the refractories of the refractory plates are exposed to hot molten steel, they wear out rapidly. Accordingly, refractories are changed every several hours of casting. Changing refractories is troublesome: the surface pressure has to be lifted, the refractory plates have to be opened, the machine bolts and the cotters have to be removed to remove the refractories, new refractories have to be installed with the machine bolts and the cotters, and the surface pressure has to be effected again. If the surface pressure is too low, molten steel leaks from between the refractory plates. If the surface pressure is too high, the slide gate does not work due to too large sliding resistance and molten steel in the vessel cannot be poured in the mold. Therefore, it is very important to effect proper surface pressure between the refractory plates.
Accordingly, various surface-pressure application devices have been developed so far, for example, to prevent human errors or to prevent the difference in workmanship among individual workers from affecting the surface pressure. Representative devices are one to press coil springs with a linkage (Japanese Examined Utility Model publication No. 17497/S59 (1984)), one to press coil springs with bolts (Japanese Unexamined Patent publication No. 132432/S54 (1979)), and one to effect surface pressure with an actuator (Japanese Unexamined Patent publication No. 115965/H5 (1993) or 169213/H5 (1993)).
The above devices of the prior art are useful in their own ways, but not necessarily satisfactory.
In case of the device of the Japanese Examined Utility Model publication No. 17497/S59 (1984), the surface pressure can be effected just by turning a lever and, hence, the work can be made in a very short time. However, when it is adopted for a large-size slide gate which requires a surface pressure as large as several to ten odd tonf, the turning torque of the lever becomes as high as several hundred Nm. If the lever is short, a single worker cannot turn it. If it is long enough for a single worker to turn it, a large space has to be secured around it. Besides, when the surface pressure is lifted, the energy accumulated in the coil springs is released at a stroke, causing a large shock, which is unpleasant to the worker and can be dangerous.
In case of the device of the Japanese Unexamined Patent publication No. 132432/S54 (1979), by using power tools, the working time and the working space required can be reduced and labor can be saved. However, power tools are costly, they require a power source, and their noise affects the working environment. If a power source is not available, the bolts can be loosened and tightened with spanners, which however takes a long time.
In case of the devices of the Japanese Unexamined Patent publication Nos. 115965/H5 (1993) and 169213/H5 (1993), the surface pressure can be effected and lifted just by operating a valve for switching the actuator. Therefore, the pressure-effecting and lifting work itself is simple and can be made in a very short time. Besides, the valve operation can be automatized. However, since such an actuator and a valve, and a power unit for the actuator are necessary, the whole device becomes costly.
If cotters are used for installing refractories, some of them may loosen while the slide gate is in service. Machine bolts do not get loose, but threads may seize up, posing a problem to the change of refractories.